1. Field of the Invention
This invention relates to electron beam devices and, more specifically, to multiple electron beam devices for converting analog electrical input signals into coded electrical digital output signals, and the like.
2. Description of the Prior Art
For a number of reasons it is preferable to manipulate electrical signals that are in coded digital form, rather than to process analog signals, in which the amplitudes of the analog signal voltages generally correspond to the respective values of the related digital sequences. Thus, a system of two electrical voltages, in which one voltage is ascribed a value of "zero" and the other voltage is accorded a value of "one", can be used with great efficiency to express numerical values in computers and in other automatic data processing equipment. Numerical values translated into this one-zero system usually are referred to as binary numbers in contrast to the "base ten" system that ordinarily is employed outside of the field of automatic computation. As might be expected, several binary coding systems, including a system known as "the Gray code," have been developed to express base ten numbers in the form of digital sequences of ones and zeroes.
There is then the difficult technical problem of swiftly and accurately converting signals, in which voltage amplitudes are related to respective numerical values or to magnitudes of some physical observation, into individual sequences of digital pulses that embody, in the particular code, the numerical value of the electrical analog input.
For many years, a considerable effort has been expended to apply cathode-ray tube principles to this task of analog to digital signal conversion. For example, as described in more complete detail in the article titled "Logical Detenting in Cathode-Ray Coding Tubes" by Bernard Lippel, Institute of Radio Engineers Transactions on Instrumentation, March, 1958, I-7, pages 29 through 37, a cathode-ray coding tube has a glass envelope to preserve a vacuum for a flat sheet of electons that are emitted from a "gun" within the envelope. The analog voltage is applied to this electron beam through a pair of deflection plates that are spaced from each other on opposite sides of the beam. The electrical field established by the analog voltages on the deflection plates cause the sheet of electrons from the gun to sweep through angles that are generally proportional to the magnitudes-or amplitudes-of the applied input signals. An apertured mask is spaced from the gun and the deflection plates in order to selectively obstruct the electron beam that is sweeping in response to the analog signals.
The apertures or perforations in the mask form a pattern that corresponds, for example, to the Gray code. In this respect, for any given angular electron beam displacement relative to the deflection plates, the particular array of perforations and electron-blocking solid portions of the mask is the digital equivalent, in the Gray code, of the amplitude of the analog voltage on the deflection plates that produced the specific angular displacement of the sheet of electrons in question. The electrons that pass through the mask apertures impact on an output signal collector, and it is this collector signal that is the desired Gray code digital output. A number of output signal collectors have been proposed for this application of which photo-electric cells that respond to light emitted from phosphors deposited on the face of the cathode-ray tube, wire grids, and the like are typical.
In contrast to the signal processing speeds that characterize present-day signal processing techniques, however, these cathode-ray tube analog to digital signal converters do not have sufficiently fast responses to justify their use. Some of these proposals, moreover, require voltages to accelerate the electrons in the beam that can be as high as 10,000 volts.
There is, moreover, a further important consideration that bears on this problem of speed in signal conversion. For a number of reasons, signals of the type generated by analog to digital conversion circuits are not steadily or continuously applied to the balance of the signal processing circuits. These digital signals are, instead, periodically "sampled" at extremely high frequencies. Typically, these signals might be sampled as many as one hundred million to four hundred million times per second. Consequently, for a sampling rate of 100 million cycles per second an interval of only 12.5 picoseconds is available to sample all of the digits in an eight digit code if an error not greater than plus or minus one digit is permissible. In the circumstances, an acceptable electron beam analog to digital signal converter must generate the necessary output signal at the proper voltage level many times in each second, and in each instance the time in which this signal can be generated is very short.
Consequently, there is a need for a high speed electron beam analog to digital converter that produces accurate signal conversion with speeds that are significantly greater than those which have been heretofore attainable.
Accordingly, it is an object of the present invention to increase signal conversion speed to an extent that justifies the use of an electron beam analog to digital signal converter.